1. Field of the Invention
The present invention relates to a semiconductor package and manufacturing method thereof. More particularly, the present invention relates to a micro-machine package having a sealed hollow chamber.
2. Description of the Related Art
Micro-machines such as micro-machine sensing elements and micro-mirrors have been widely researched in the recent years. These micro-machines have miniaturized movable or oscillatory structures including cantilever, capacitor devices, yokes and hinges. In general, semiconductor devices such as a complementary metal-oxide-semiconductor (CMOS) may serve as control elements to trigger the movement of the oscillatory structure inside the micro-machines. Since the action produced by the micro-machine depends on the movement of the miniaturized oscillatory structure, it is critical that the package enclosing the micro-machine should not touch any part of the miniaturized oscillatory structure under all circumstances.
A number of package structures for micro-machines has been disclosed, for example, disclosed in U.S. Pat. No. 6,415,505 which is incorporated herein for reference. FIG. 1 is a schematic cross-sectional view of a conventional micro-mirror package. As shown in FIG. 1, the micro-mirror package 10 comprises a micro-mirror chip 14 having an oscillatory structure 15 and a semiconductor chip such as a complementary metal-oxide-semiconductor (CMOS) chip 12. The micro-mirror package 10 also comprises an actuator device or electrode 13 that can correspond with the oscillatory structure 15. The micro-mirror chip 14 is disposed above the CMOS chip 12 but uniformly set apart from the CMOS chip 12 by spacers 16 to form a sealed hollow chamber 30. Thus, the oscillatory structure 15 is free to move within the hollow chamber 30 and coordinates with the electrode 13. The CMOS chip 12 is fixed to a ceramic substrate 20 through an adhesive layer 32. Through a plurality of bonding wires 36, the CMOS chip 12 is electrically connected to the ceramic substrate 20. The ceramic substrate 20 has a circular wall 22 surrounding the CMOS chip 12. A transparent lid 26 is fastened to the top of the circular wall 22 through a sealing element 24. Furthermore, the micro-mirror chip 14 may connect with the substrate 20 through a ground wire 34.
In the aforementioned structure, the CMOS chip 12 is fabricated using silicon and the micro-mirror chip 14 is fabricated using transparent glass such as the brand name Eagle 200 brand of Corning Inc. In general, the coefficient of thermal expansion (CTE) of the glass is very close to the CTE of silicon, about 4 ppm/° C., so that the thermal mismatch is minimized. However, the CTE of the substrate 20 is about 12˜14 ppm/° C. and a significant difference in CTE exists between the substrate 20 and the CMOS chip 12. If the micro-mirror package 10 encounters any temperature changes or cycles during the fabrication process or during normal operation, the CMOS chip 12 will be subjected to the thermal stress, which can easily lead to warpage or fatigue failure. In particular, if the micro-mirror package 10 is an optical device installed within a projector, minute deformation in the CMOS chip may lead to a substantial drop in image quality. Hence, it is essential to provide a semiconductor micro-machine package structure capable of minimizing heat-related deformation.